1. Field of the Invention
The present invention relates to a semiconductor device and, more particularly, to a semiconductor device having a MOS transistor designed to prevent damage to the insulating film of a gate electrode in a plasma process and the like.
2. Description of the Prior Art
Conventionally, many plasma processes are used in a semiconductor device manufacturing process. In a plasma process, a voltage is applied to an electrode. This damages a gate insulating film of a MOS device, and results in a decrease in the yield of LSIs or a deterioration in reliability. This problem will be described below by taking a transistor for checking/measuring device characteristics as an example.
FIG. 1 is a plan view of a measurement transistor. A measurement transistor has a diffusion region formed in that region on a P-type substrate 1, which is surrounded by a field insulating film 2. A gate electrode 5 is formed across this diffusion region. Regions adjacent to the gate electrode 5 in the diffusion region serve as heavily-doped N-type diffusion regions 3A and 3B. Probe pads 7A, 7B, and 7C are respectively connected to contacts 6 formed on the gate electrode 5 and the heavily-doped N-type diffusion regions 3A and 3B.
The pads 7A, 7B, and 7C are respectively used for gate, source, and drain electrodes. During plasma etching of the pad electrode, charge flows into the gate electrode 5 through each pad, resulting in a deterioration in the gate insulating film. Since the area of each pad is relatively large with the length of one side ranging from 50 .mu.m to 100 .mu.m, damage caused by the plasma cannot be neglected.
As shown in FIG. 2, a protective diode D may be formed to prevent charge buildup on the gate electrode 5 through the pad 7A. Referring to FIG. 2, the pad 7A for the gate electrode is connected to a heavily-doped N-type diffusion region 3D through an interconnection 9D. Since the P-type substrate 1 is located below the heavily-doped N-type diffusion region, the gate pad 7A is connected to the heavily-doped N/P-type diode D. As a consequence, the charge built up on the gate electrode 5 can be released to the P-type substrate 1.
A structure in which the diode or transistor of an input/output protective circuit is made of polysilicon is disclosed in Japanese Unexamined Patent Publication No. 1-253276.
As the size of the semiconductor device decreases, however, the thickness of the gate insulating film is reduced, and the breakdown voltage of the gate insulating film gradually decreases. On the other hand, the breakdown voltage of the protective diode hardly changes because the voltage is determined by the impurity concentrations of the diffusion region and its adjacent well. For this reason, when the thickness of the gate insulating film becomes about 8 nm or less, the breakdown voltage of the protective diode becomes higher than that of the gate insulating film. The diode having the conventional structure cannot therefore provide sufficient protection.
This state will be described below with reference to FIG. 3.
FIG. 3 shows the relationship between "gate voltage" and "gate current" and the characteristics of a diode when a thick gate insulating film is formed. Referring to FIG. 3, the abscissa represents the gate voltage with reference to the substrate voltage. A current flowing in the gate insulating film is tunneling current and abruptly increases at a given voltage or higher. Upon application of a positive voltage, the diode is reverse-biased. Hence, a breakdown occurs at a given voltage, and the current flowing in the diode abruptly increases. In a plasma process, plasma light generates carriers in the substrate. As a result, a leakage current that does not have much dependence on voltage flows. This behavior is expressed as diode characteristic in FIG. 3.
Assume that Vbd (thick film), Vbd (thin film), and Vbd (diode) respectively represent the breakdown voltage of the gate insulating film (thick film), the breakdown voltage of the gate insulating film (thin film), and the breakdown voltage of the diode. When the gate insulating film is thick, Vbd (diode)&lt;Vbd (thick film). When, therefore, the gate electrode is connected to the diode, the characteristic represented by a dashed line (c) in FIG. 3 is obtained. The current flowing from the plasma and the gate voltage have a predetermined relationship. This characteristic is represented as the plasma current (dotted line) in FIG. 3. The plasma current does not have much dependence on the gate voltage and can be approximately regarded as a constant current. Since most of the current from the plasma flows into the diode, no damage occurs.
As the thickness of the gate insulating film is reduced, the breakdown voltage decreases, and Vbd (diode)&gt;Vbd (thin film). As a consequence, the characteristic represented by the dashed line (b) in FIG. 3 appears, and most of the current from the plasma flows into the gate insulating film, resulting in damage to the gate insulating film. That is, the thin gate insulating film cannot be effectively protected by the diode in the prior art.